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- Legacy information systems (LIS) are typically the backbone of an organization’s information flow and the main vehicle for consolidating business information. - They are mission critical, and their failure can have serious impact on business. - Definition: Any information system that significantly resists modification and evolution. - They can cause host organizations several problems: -LIS usually run on obsolete hardware that is slow and expensive to maintain. -Software maintenance can also be expensive: because documentation and understanding of system details is often lacking, tracing faults is costly and time-consuming. -A lack of clean interfaces makes integrating LISs with other systems difficult. -LISs are also difficult, if not possible, to extend. Overview

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- Several solutions have been proposed: -Redevelopment (Big Bang approach): Rewriting existing applications -Wrapping: Wrapping an existing component in a new, more accessible software component -Migration: Moving LIS to a more flexible environment, while retaining the original system’s data and functionality. Overview

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- Each approach varies in terms of changes required and costs and risks involved. Redevelopment leads to the most changes (system revolution) and wrapping the least (system evolution). LIS Coping Strategies

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Known as Big Bang approach, Chicken Little approach or Cold Turkey approach. In reality, the risk of failure is usually too great for organizations to seriously contemplate a redevelopment approach. Another very real concern stems from the fact that technology and business requirements are constantly changing. Thus, at the end of a long process, an organization might find itself with a redeveloped system based on obsolete technology that no longer meets its business needs. Redevelopment

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Most practical solutions focus on wrapping, which surrounds existing data, individual programs, application systems, and interfaces with new interfaces. In essence, this gives old components new operations or a ‘new and improved’ look. The wrapped component acts as a server, performing some function required by an external client that does not need to know how the service is implemented. Wrapping

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Although it is a much more complex undertaking than wrapping, if successful, migration’s long-term benefits are also greater. Before embarking on a migration project, engineers, management, and users should undertake an intensive study to find the most appropriate approach for solving their organization’s LIS problems. Database population Migration

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Up to 80% of a migration engineer’s time could quite legitimately spent testing the target system. Given the legacy system’s mission-critical nature, target system outputs must be completely consistent with those of the LIS. Thus, it is inadvisable to introduce new functionality to the target system during the migration project. However, on a practical level, migration projects are often expected to add functionality to justify the project’s expense and risk. In this case, the LIS should be migrated first. New functionality can be incorporated into the target system after the initial migration. Migration: Testing and Functionality

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The last step in the migration project is the cut-over from the LIS to the target system. 3 different transition strategies have been proposed: The cut-and-run strategy consists of switching off the LIS and turning on a new feature-rich replacement. With the phased interoperability strategy, the cut-over is performed in small, incremental steps: each step replaces a few LIS components (applications or data) with corresponding target components. In the parallel operations strategy, LIS and target systems operate simultaneously, with both systems performing all operations. During this period, the target system is continually tested; once it is fully trusted, the LIS is retired. Migration: Cut-Over

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1. The Chicken Little Strategy Michael Brodie and Michael Stonebraker propose this strategy, which lets LIS and target systems interoperate during migration using a mediating module, generally known as a “gateway”. The Chicken Little strategy offers an 11-step plan for the cutting over from the LIS to the target system. Each step is incremental: Analyze the LIS Decompose the LIS structure Design the target interface Design the target application Design the target database Install the target environment Create and install necessary gateways Migrate the legacy databases Migrate the legacy applications Migrate the legacy interfaces Cut over to the target information system. Migration Methods

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1. The Chicken Little Strategy The target system is intially quite small, but grows as the migration progress. During migration, the LIS and target systems form a composite information system. Chicken Little uses a forward gateway to translate and redirect calls to the tarhet database’s results for use by LIS applications. A reverse gateway maps the target data to the LIS database. This mapping can be complex and slow, thus affecting new applications. The Chicken Little approach can use data duplicated accross the LIS and target databases. To maintain data integrity and consistency, a transaction coordinator intercepts all update requests from LIS or target applications and identifies whether they refer to data replicated in both databases. If they do, the update is propogated to both databases using two-phase commit protocol. Although this strategy has the advantage of breaking the migration process into a series of well-designed stages, it can involve highly complex strategies to ensure consistency between the target and LIS databases. Migration Methods

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2. Butterfly Strategy It assumes that although the LIS must remain operable throughout migration, the LIS and target system need not interoperate during the process -> This assumption eliminates the need for gateways and their potential complexity. The Butterfly methodology focuses on LIS data migration, and develops the target system in an entirely seperate process. Once data migration commences, the LIS data store is set to read-only. The data access allocator redirects LIS data manipulations; the results are stored in a series of auxilary data stores, or TempStores. Migration Methods